msm: kgsl: a2xx: Check the right interrupt status bits

Check the actual interrupt status bits in a2xx_irq_pending instead
of reading the mask registers which were always set and returning
true. Troutslap for the original author (me).

CRs-fixed: 467381
Change-Id: Ic0dedbada1c5fa64548221718c1eaf9b6eaaa1e8
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>